1. Field of the Invention
This invention relates to a rewritable type optical disc such as DVD-RW, a manufacturing method and a manufacturing system of such an optical disc.
2. Description of Related Art
Since a variety of CD-ROM formats have been proposed and DVD (Digital Versatile Disk) has been developed, there are various types of optical disks, and the number of the type of the optical disks is still increasing. At present, the CD-ROM formats for read-only type CD-ROM and rewritable type CD-ROM have been proposed, and there are various DVD formats for read-only type DVD-Video, rewritable type DVD-RW which can be rewritable for limited times, and DVD-RAM which can be readable and writable for unlimited times.
In order to avoid the illegal copy of contents recorded on the read-only type DVD-Video to a DVD-RW in a bit-by-bit manner, a DVD-RW in conformity with the DVD-RW version 1.0 standard, currently on the market, has embossed pits formed in the control data zone. Those embossed pits are called as unreadable emboss and have the same depth as the grooves. By forming the embossed pits in the control data zone, reading out the data illegally recorded in the control data zone of the DVD-RW is prevented.
The above-mentioned measure to prevent the illegal copy has such a problem that the embossed pits put the adverse affects on the reproduction of the DVD-RW. In this view, in a DVD-RW in conformity with the next DVD-RW version 1.1 standard, the control data is recorded as embossed pits having depths deeper than the groove tracks, and the control data zone is formed as the read-only area.
However, when deep embossed pits are formed, the groove tracks must be formed on the original optical disk 80 by exposing the resist layer to its intermediate depth by a weak light beam B as shown in FIG. 8A. The bottom shape of the grove track of the DVD-RW thus formed is not necessarily flat, and hence there is a basic problem that desired recording/reproduction characteristics cannot be achieved.
In FIGS, 8A, 8B and 8C, the light beam for forming the embossed pits is shown as the light beam A and the light beam for forming the groove tracks is shown as the light beam B. FIG. 8A shows the exposing state of the original recording disc in a sectional view. FIG. 8B shows the sectional view of the DVD-RW after the development, and FIG. 8C shows the sectional view of the DVD-RW manufactured from the developed original recording disc. FIGS. 8A to 8C are shown to illustrate the bottom shape of the DVD-RW in each phase. In FIG. 8C, the multi-layer 84, the reflective layer 85, the adhesive layer 86 and the protection layer 87 are formed on the transparent substrate 81 in this order. As seen, the deep embossed pit 82 formed on the transparent substrate 81 of the DVD-RW has a relatively flat bottom shape. In comparison, the bottom shape of the groove track 83 formed on the transparent substrate 81 is not flat, resulting in the degradation of the groove shape.